The present invention relates to optical phase-shifters, particularly to optical phase-shifting interferometry, and more particularly to a constant volume gas cell optical phase-shifter which can produce motions which are accurate to about 5 xc3x85.
Optical phase-shifting is utilized in a variety of applications, and particularly in interferometry. Present commercial technology for phase-shifting interferometry uses three piezoelectric transducers (PZTs) on a mirror, or a long fiber optic cable that is stretched by a piezoelectric transducer. In the PZT/mirror arrangement, for example, A xcfx80/4 phase step at the optical wavelength of 5320 xc3x85 requires that the mirror be longitudinally displaced by 332.5 xc3x85. These motions can be made accurately enough for routine optical fabrication. However, in order to do interferometry to an accuracy of 1 xc3x85 in the surface height, these motions must be accurate to about 5 xc3x85, and such accuracy is very difficult with existing technology.
The present invention provides a phase-shifting arrangement that can easily achieve the accuracy required for interferometry, and is inexpensive to fabricate. The invention involves a constant volume gas cell optical phase-shifter which has the capability to make the required phase steps accurately. The constant volume gas cell can make very precise phase steps that are extremely constant across the beam wavefront. As a beam travels thorough a gas filled tube of the cell the length of the tube can be changed, thus changing the optical path length of the beam traversing the cell. Because the cell is constant volume, the pressure, temperature, and density of the contained gas do not change as the cell changes length. This means that there is an exactly linear relationship between the changes in length of the gas cell and the change in optical phase of the beam traversing it. The constant volume gas cell phase-shifter is simple to construct and provides very precise phase steps as the length of the cell is changed.
It is an object of the present invention to provide an improved optical phase-shifter.
A further object of the invention is to provide an optical phase-shifter having an accuracy compatible for interferometry applications.
A further object of the invention is to provide an optical phase-shifter which is of simple construction, but which produces high accuracy.
Another object of the invention is to provide an improved optical phase-shifter which involves a constant volume gas cell.
Another object of the invention is to provide a constant volume gas cell which can make very precise phase steps that are extremely constant across a beam wavefront.
Another object of the invention is to provide a constant volume gas cell optical phase-shifter wherein precise phase steps are accomplished by changing the length of the gas cell through which a beam traverses, the changing the optical path length of the beam.
Other objects and advantages of the present invention will become apparent from the following description and accompanying drawings. The invention is directed to a constant volume gas cell optical phase-shifter which can make very precise phase steps that are extremely constant across the beam wavefront. The cell is basically composed of three concentric tubes, two of which are interconnected and positioned such that the inner of the interconnected two tubes moves within the third tube and the third tube moves within the outer of the interconnected two tubes, with each tube being provided with a seal at one end thereof, and with an equalizer passage between the third and outer tubes, and between the two connected tubes and the exterior of the cell. The inner tube and the third tube and a portion of the outer tube are filled with a gas, such as air, for example, at a pressure different than atmospheric. The inner and third tubes are provided with optical windows at an end opposite the seals. The gas within the three tubes is at a constant volume and passes from the third tube to the outer tube or vice versa upon the third tube being moved with respect to the connected inner and outer tubes, thus the inner and third tubes are full of gas at all times, and the volume of gas in the outer tube changes as the third tube and interconnect two tubes move with respect to one another. Thus, with a beam, such as a laser beam, traversing the third and inner tubes of the cell, as the length of the cell changes (two move with respect to each other), the optical path length of the laser beam traversing the cell changes. Since the gas in the cell is at a constant volume, there is an exact linear relationship between the change in length of the gas cell and the change in optical phase of the laser beam traversing it. Thus, the cell can be designed, for example, such that a change or motion of 10 millimeter in cell length cause one wavelength change in the optical path. Thus, this motion in cell length can be made to great wavelength fractional accuracy, such as required by phase-shifting interferometry for its phase step.